1. Field of the Invention
The present invention relates to a choke. More particularly, the present invention relates to a choke having a relatively small height and size.
2. Description of Related Art
A choke is used for stabilizing a circuit current to achieve a noise filtering effect, and a function thereof is similar to that of a capacitor, by which stabilization of the current is adjusted by storing and releasing electrical energy of the circuit. Compared to the capacitor that stores the electrical energy by an electrical field (electric charge), the choke stores the same by a magnetic field.
In the past, the chokes are generally applied in electronic devices such as DC/DC converters and battery chargers, and applied in transmission devices such as modems, asymmetric digital subscriber lines (ADSL) or local area networks (LAN), etc. However, in recent years, with development and demands of electronics technology, various electronic products are continually developed, and have a general trend of lightness, slimness, shortness and smallness. The chokes are widely applied to information products such as notebooks, mobile phones, LCD displays, and digital cameras, etc. Though, a height and size of the choke can be a problem in utilization.
FIG. 1 is a cross-sectional view of a conventional choke. Referring to FIG. 1, the choke 10 has a coil 12 and a magnetic material 14 encapsulating the coil 12, wherein a shape size of the choke 10 is above 4 mm×4 mm, and a height thereof is above 2.5 mm. A method of fabricating the choke 10 is as follows. First, a wire is winded into the coil 12, and the winded coil 12 is disposed in a mold. Next, the magnetic material 14 fills in the mold for encapsulating the coil 12, wherein the magnetic material 14 is, for example, insulated magnetic powder with particles. Next, a pressure molding and a firing process are performed to form the choke 10.
In the fabrication process of the choke 10, since the magnetic material 14 has the particles, and the coil 12 is a hollow structure, during the pressure molding process, the particles of the magnetic material 14 can press the coil 12 under the pressure, so that the coil 12 can be cracked or deformed. Moreover, if the height of the choke 10 is reduced to be less than or equal to 2.5 mm, a relatively fine wire (especially having high inductance) can be applied to wind the coil 12. However, the coil 12 winded by such fine wire has a poor strength, and the pressure molding cannot be performed, so that reduction of the size of the choke 10 cannot be implemented.
FIG. 2 is a cross-sectional view of another conventional choke. Referring to FIG. 2, the choke 20 disclosed by the U.S. Pat. No. 7,209,022 includes a drum-core 30, a wire 40, an exterior resin 50, and a pair of external electrodes 60. The drum-core 30 includes a pillar 32, a top board 34 and a bottom board 36, and the pillar 32, the top board 34 and the bottom board 36 form a winding space S. The wire 40 is winded around the pillar 32, and is located in the winding space S. The exterior resin 50 fills in the winding space S, and encapsulates the wire 40, wherein the exterior resin 50 is coated by a dispenser, and a glass transition temperature that the exterior resin 50 is transited from a glass state to a plastic state is below −20° C. The pair of external electrodes 60 is disposed on a lower surface of the bottom board 36.
Since the exterior resin 50 of the choke 20 has a volatile solvent, and is a mixed material formed by a plurality of formulations, after it is coated in the winding space S, it has to be rested at a room temperature for 30 minutes to vaporize the solvent, so as to perform a heat-curing process. Therefore, a fabrication time of the choke 20 is relatively long. Moreover, since the exterior resin 50 is formed by a plurality of the formulations having the solvent, and the glass transition temperature is below −20° C., a pot-life and a heat time of the exterior resin 50 are influenced by a formulation ratio, so that the pot-life of the exterior resin 50 is shortened, and a part of the formulations cannot be used for a mass production.